I have the exact same problem (ATI AIW 9700pro). Unfortunately, I never got it fixed. The problem started with Windows XP service pack 2... they apparently changed how the UART memory manager worked and it seems to totally break systems with AMD processors + AGP based video cards. I tried getting linux to work and it had the same problem (with accellerated grafics). ATI and MS both suggest downloading the latest BIOS firmware and then applying the latest ATI grafic card drivers. It very well could be bad system memory (i've tried memory checks), but I blame Win SP2... I havn't bothered with swapping out the memory, with prices nowadays, it might be worth trying...

So, If you ever resolve this, let me know, I'd be interetsed... (because now I can't seem to get any AGP grafics card to work)

FYI, I have a ASUS Dragon Ultra motherboard...